There are essentially 4
stages of pregnancy in cattle; Fertilization,
Cleavage, Implantation and Gestation.
These processes are the most important factor in relation to failure of a cow
or heifer becoming pregnant, therefore it is important to understand these
processes in order to minimise embryonic
loss and improve the total yearly
calving percentage.

Stage 1 –
FERTILIZATION

The entire process of sexual reproduction is centred on the
act of fertilization; essentially
this act is the fusion of two cells
(the male sperm cell and the female egg), to form one single cell, known as the zygote.

Cow sex cells

Cow body cell

Each sperm cell or egg cell contains half the normal number of chromosomes (chromosomes are the
structures of DNA, genetic material, that are held within the nucleus of
cells). All body cells contain a full set of chromosomes and cattle have 30
pairs (so 60 individual chromosomes altogether) in a normal adult cell; sex
cells contain half this number, so
cattle sex cells have 30 individual chromosomes. The main reason for this is so
when the egg and sperm fuse, the resulting zygote contains a full, and not
double, set of chromosomes. Cells that contain a full set are known as diploid and those that contain half are
called haploid.

For example, using the diagram above, the orange and green
cells (sex cells) are haploid cells
– they contain half the genetic number of chromosomes. If they fuse together to
create a zygote, the zygote will
contain the correct number of chromosomes (30 pairs) and can go on to form an
embryo.

In cows, fertilization takes place at the ampullaristhmic
junction of the oviduct (this is the stretch of fallopian tube between the
ampulla and the isthmus).

The female egg, which was released from the ovarian follicle surrounded by cumulus cells at the time of ovulation,
arrives at the junction 1 -2 days after standing heat. The egg is denuded of
the cumulus cells and ready for
fusion with the sperm. Cumulus cells are follicular cells that protect the
egg up until it is ready to be met with a sperm. The fertile life span of the
ovum is very short (8 – 12 hours).

Sperm are deposited in the vagina during natural service
(mating with a bull) and into the uterus during artificial insemination. Although the total number of sperm
deposited into the female tract measures in the thousands of millions, the number travelling as far as the ampulla
is probably not much more than 1000 in any mammal. Some sperm reach the site of
fertilization very quickly (15 minutes) but in order to fertilize the egg,
sperm must go through changes termed capacitation.
In cattle, sperm capacitation takes about 4
hours. The fertile life span of sperm is in the range of 30 – 48 hours.

The sperm must penetrate two membranes
on the egg in order to complete fertilization. The first outer membrane that
the sperm must penetrate is called the zona
pellucida. Passage of the sperm into the zona pellucida is facilitated by proteolytic enzymes (chemicals which
break down the components of the membrane) which are released from the sperm
head when the acrosome is lost.

The zona pellucida
undergoes some sort of change after the passage of a sperm, which makes it less
easy for subsequent sperm to enter. This is called the zona reaction and is one measure of protection against polyspermy.

The last stage of sperm penetration into the egg involves the attachment of the
sperm head to the surface of the vitelline
membrane. This is a vital period in the fertilization process since it is
at this time that activation occurs.
Stimulated by the close proximity of the sperm, the egg awakens from its
dormancy and development begins. The other defence
mechanism against polyspermy
(more than one sperm fertilizing the egg) is shown by the vitelline membrane and is termed vitelline block.The fertilizing sperm is actively engulfed by the vitelline; but subsequently,
the vitelline surface becomes unresponsive
to sperm contact and no further sperm are engulfed.

The reason for the changes to the membranes of the egg,
which only allow one sperm to reach the egg, is that polyspermy leads to an abnormal
number of chromosomes in the embryo, which is a fatal condition.

Once the male and
female membranes fuse, the genetic material from each parent is able to
join in a process called syngamy to
form one new individual. The process of fertilization is now complete and the fertilized ovum undergoes its first
cleavage to produce a two celled embryo.
Each daughter cell now contains the normal diploid
number of chromosomes, half of which have been derived from the egg and
half from the sperm. The duration of fertilization from the time of penetration
of the sperm to the first cleavage is estimated to be around 20 – 24 hours in cows.

Male sex cell (sperm) with 30 chromosomes

Zygote with 60 chromosomes

Female sex cell (ovum) with 30 chromosomes

STAGE 2 – CLEAVAGE

Cleavage of the newly fertilized embryo is simply mitotic division of one cell
into two cells, two cells into four cells, four into eight, and so on. In each
mitotic division, the genetic information is duplicated such that each daughter cell arising from the original
cell contains exactly the same chromosomes.

The embryo moves from the oviduct into the uterus
at about the 8-cell stage, usually 3
– 3 ½ days after ovulation. At this point, the embryo is free-floating within the uterus, and depends upon uterine secretions for nourishment.

By the 16-32 celled
stage, the cells of the embryo are crowded together into a compact group
still within the zona pellucida. The
embryo is now known as a morula.
Fluid begins to collect between the cells and an inner cavity or blastocyst appears. Once the cavity
begins to expand, the embryo is known as a blastocyst.

Some embryonic loss occurs during the time of blastocyst
formation and may be due to embryonic chromosome defects. In cattle, the
blastocyst stage occurs between days 7-8
after ovulation. This is the time when embryos are flushed from a donor cow
and implanted into recipient cows during embryo
transfer.

A single layer of large flattened cells, the trophoblast layer, surrounds a knob of
smaller cells which lie to one side of the central cavity. The knob of cells,
or inner cell mass, will give rise to the adult organism while the cells of the
trophoblast will from the placenta
embryonic membranes. In the cow, the zona
pellucida sheds at about day 8
and blastocyst elongation begins a few days later.

Maternalrecognition of pregnancy must take
place about day 16 to 17 or the
uterus will produce prostaglandin F2A
(PG) which will cause the corpus luteum to regress, leading to a decline in
progesterone concentration. A lack of
progesterone is fatal for pregnancy – the embryo will not be able to
implant (which is the next stage).

Therefore in the case of a pregnancy, the embryo must
somehow signal to the mother that she is pregnant, so that the regression of
the CL can be inhibited. It has been demonstrated that bovine and ovine (cattle and sheep) embryos produce and release a specific pregnancy protein – this protein is
known as Interferon-T (INF-T). The
mechanism of luteolysis (regression of CL) inhibition is now well established…

·Oxytocin receptors on the uterine luminal
epithelium are inhibited.

·

Usually PG is stimulated to be produced by a PG synthesis hormone; in
the case of a pregnancy, an

inhibitor is
produced which blocks this synthesising hormone.

When the female cow is not pregnant

When the female cow is pregnant

Significant embryo losses can occur around the time of
maternal recognition of pregnancy due to either failure of the embryo to
produce signals or failure of the mother to recognize the signal from the
embryo. If the embryo died, or if the cow fails to recognize pregnancy, the
corpus luteum will regress normally and the cow will return to estrus at about
21 days after mating.

STAGE 3 –
IMPLANTATION

While the embryo is undergoing cleavage and blastocyst
formation, the uterus is also
undergoing changes which prepare the
way for implantation. The embryo is said to be implanted when it becomes fixed in position and physical contact with the mother is
established. Implantation in ruminants (including cattle) is non-invasive and some people prefer to
use the term attachment – as there
is close attachment between embryonic
membranes (i.e. membranes of the foetus) and the endometrium overlying caruncles (i.e. membranes of the maternal
uterus). Shortly thereafter, the placenta is established.

Ruminants have a cotyledonary placenta – instead of having a single large area of contact between
maternal and foetal vascular systems (points where blood/serum can be
exchanged), these animals have numerous
smaller placentae. The terminology used to describe ruminant placentation
is:

·Cotyledon:
the foetal side of the placenta

Caruncle: the maternal side of the
placenta

·Placentome:
a cotyledon and caruncle together

Caruncles are oval or round thickenings in the uterine
mucosa resulting from proliferation of sub
epithelial connective tissue. As shown in the image below, caruncles are
readily visible in the non-pregnant uterus. Further, they are the only site in
the uterus to form attachments with
the foetal membranes. Patches of chorioallantoic membrane become cotyledons by developing villi that extend into crypts in the caruncularepithelium.

The image below shows caruncles in an incised non-pregnant sheep uterus (left)
and cross sections through placentomes from a mid-gestation sheep (right).
Bovine placentomes look similar, but have a convex appearance rather than the
concave shape seen in sheep…

Pregnant sheep, goats and cattle have been 75
and 125 placentomes. The image below shows an incised uterus from a pregnant sheep roughly 50 days of gestation.
The numerous button-like structures
are placentomes and the surfaces in view are actually cotyledons. The slightly milky-looking membrane covering and
between placentomes is the chorioallantois.
The foetus is clearly visible inside the amnion.

The image below shows a bovine conceptus
dissected away from the uterus. The size of the chorioallantois relative to the amnion and foetus is evident. The
cotyledons are readily observed; caruncles have been left behind with the
uterus. Cattle foetus are located in one uterine horn, the large
chorioallantois fills both uterine horns
and placentomes are present throughout the uterus.

During parturition, there is substantial loosening of the cotyledonary villi
from caruncles, and the placentomes
expand laterally. After expulsion of the foetus
and loss of foetal circulation to the
cotyledons, capillaries within the
villi collapse, leading to a decrease in their size. The uterus contracts
and the caruncular stock shrinks,
further enhancing the separation of
cotyledons from caruncles. In the normal case, foetal membranes with cotyledons are delivered within 12 hours
from birth. However, in many cases which are still considered normal, the
membranes will not be passed up until the 10th day after calving, at
which time almost all cows will expel the membranes. Failure to do so can lead to detrimental infections and cycling problems.

The images below show what the expulsion of membranes should look
like…

Has your cow still not passed her foetal membranes? Does
your cow have a temperate post calving? These problems are more often than not
associated with membrane retention - For more information on post pregnancy
processes and care, follow this link to my specific blog post… *insert link*

Structure of the
Placenta…

A prominent feature of the ruminant placenta is the presence
of large numbers of binucleate cells
(cells with two nuclei). These cells arise early as part of the foetal trophoblast from cells that fail to
undergo cytokinesis (the separating of one dividing cell, into two separate
cells) following nuclear division. They invade
and fuse with caruncular epithelial
cells to from small syncytia
(multi-nucleated cells forming specialised tissues). Binucleate cells secrete placental lactogen which can stimulate
mammary cell growth and milk secretion.

Ruminants basically have an epitheliochorial placenta, but because the uterine epithelium is
modified by invasion and fusion of binucleate cells, its structure if generally
referred to as synepitheliochorial.

“Epitheliochorial placenta:

The uterine epithelium
of the uterus and the chorion are in contact in this placentation, and there is
no erosion of the epithelium. It is characteristic of cows, sows and mares. Also
called adeciduate placenta.”

General aspects of placental transport are similar to that
seen in other species. Immunoglobulins
(antibodies) are not transported across the placenta from mother to foetus, and
therefore, apart from foetal infections should they occur, the young ruminant
is born without circulating antibodies. This is why it is extremely important
for calves to receive sufficient and continuous colostrum within the first few days of life.

The major hormones of
ruminant placentae are progesterone
and other progestins, oestrogens and placental lactogen.

Generally speaking, most cattle and goats require a corpus luteum throughout
pregnancy in order to maintain sufficient levels of progesterone. This is in contrast to sheep, which at around day 70 can maintain pregnancy even without
a corpus luteum.

In cows, the foetal placenta starts to produce placental lactogen around 4 months of
gestation and remains low throughout parturition. This hormone stimulates mammary growth and prepares the mammary gland for the
impending period of lactation – it does not stimulate milk production, it
is has more of a preparatory action than an active role.

In cattle, the embryo remains in the uterine cavity and whatever
attachment it forms with the wall of the uterus before the formation of the
placenta is of an extremely loose nature. Progesterone
secreted by the corpusluteum of the ovary acts to decrease muscular activity of the uterus. In
addition, progesterone increases by supply to the uterus and stimulates proliferation of the uterine epithelium and an increase in
uterine milk secretions.

By 33 days
post-mating, the foetal chorionic membrane has formed a fragile attachment
with two to four of the cotyledons surrounding
the foetus; within a few days, maternal caruncles and foetal cotyledons have
become so intimately interdigitated that
the embryo is being completely nourished through the cotyledons. Growth of
the cotyledons is also stimulated by progesterone.
The other significant loss of embryos occurs at the implantation stage. Cows
losing embryos at this time will return to oestrus approximately 40 – 42 days post mating.

STAGE 4 – GESTATION

The length of gestation extends from the time of fertilization until birth of the offspring. On average,
gestation lasts for 283 days in cattle
(ranging from 279 – 285 days).

Changes occur in the female’s reproductive tract as
pregnancy progresses. The foetal and
maternal hormone systems interact throughout pregnancy such that pregnancy
not only is maintained but that
continued development and growth of the foetus is assured. Perhaps the most
marked example of the ability of the foetus to regulate the mother’s system is
its ability to program development of the udder so that milk production is synchronized with parturition (giving birth).

The foetal placenta is the major unit producing hormones during pregnancy, and the foetus plus placenta
contribute to hormonal changes in the mother’s circulation in late pregnancy.
Hormone production by the foetus may also affect its own growth and regulate maternal function to assure adequate
environmental conditions (I.e. oxygen, nutrients, water and minerals).

Swelling (oedema) and
an increase in blood supply are the major reactions of the vulva to pregnancy. The oedema
increasing with the progress of pregnancy. These vulval changes occur around
the fifth month in heifers and the
seventh month in cows. The lining of
the vagina is pale and dry during
most of gestation but becomes swollen
and pliable toward the end of pregnancy.

During pregnancy, cervicalsecretions increase to produce very viscid mucus, serving to seal the
cervical canal by the so called ‘mucous plug’ of pregnancy. Before parturition,
this seal breaks down and is discharged as strings. During pregnancy, the
external orifice of the cervix remains tightly
closed. A few days before the onset of labour, relaxin is released by the CL of pregnancy; along with the
increasing levels of oestrogen,
acting to relax the cervix and pelvis.

As pregnancy progresses, the uterus undergoes gradual enlargement to permit expansion of the
foetus, but its muscular walls remain quiet to prevent premature expulsion. Three phases can be identified in the adaption
of the uterus to accommodate pregnancy – proliferation,
growth and stretching. Uterine proliferation occurs before
blastocyst attachment and is promoted by high levels of progesterone. Characteristic changes of the inside lining of the
uterus initiated by progesterone are increased
blood supply, growth and coiling of the uterine glands and leucocyte infiltration. Uterine growth starts after
implantation. Uterine growth includes muscular
hypertrophy and an extensive increase in connective tissue. Modification of connective tissue is important
both during uterine adaption to the
growing foetus and during involution
after calving. The structural changes which take place in the pregnant uterus
are reversible but are restored at
different rates after parturition. During the period of uterine stretching
(last trimester) uterine growth diminishes
while its contents are growing at an accelerating rate.

When the mother recognizes the presence of a viable foetus
in the uterus, the CL persists as the ‘CL of pregnancy’ (CLoP) and subsequent oestrous cycles are suspended. During
early pregnancy this suspension may not be complete as considerable follicular development occurs in the
ovaries and some may even reach preovulatory
size; however, these follicles eventually become atretic (i.e. the follicle
degenerates before reaching
maturity). The CLoP in the cow persists at a maximal size and continues to
produce progesterone throughout pregnancy. However, from about day 150 – 250 of gestation, the
foetal-placental unit is capable of sustaining progesterone production and
pregnancy should the CL abnormally/prematurely regress.

Usually, animals gain
weight during pregnancy due to the growth of the foetus as well as
increases in maternal body weight. In heifers, nutrient retention due to growth
may mask actual weight gain due to pregnancy. Therefore, heifers must receive
adequate diets so that both the heifer and the foetus grow during pregnancy.

Considerable alternation in the
distribution of water occurs in pregnancy. Some of this is mechanical and is
related to the increase in venous
pressure due to the weight of the enlarging uterus. Oedema extending from
the udder to the umbilicus is
frequently observed during late
gestation in cows.

8
comments:

I do yoga regularly because I've read that it’s a great way to get your body strong, limber and ready for childbirth, but I also ride a stationary bike at home when I’m short on time or go for walks and hikes when the surf is flat, too big or too crowded. learn more

It’s my greatest joy to share my testimony for over 4 years i have be barren and now God has used [ Lady cafai treatment ] to bring joy to my home,the course of this was irregular period and menstrual pain which lead to uterus blockage of my tube ,but today there is an unending joy in my home with the help of a great native herbal Lady cafai she gave me herbs and which I apply for a short period of time which help me I conceive without no delay now i have a baby girl in my arms God is great just try and see your testimony will be greater than my.I highly recommend. https://ladycafaiherbalcenter.com/ WhatsApp messenger +15183519658